Conventionally, as a spectacles-type image display device, for example, a device that includes an image output unit held on the temple side of spectacles and an eyepiece optical unit held adjacent to a lens of spectacles is suggested. Such spectacles-type image display device is configured so that the image light of electronic images to be displayed that is output from the image output unit is incident on an eyeball of a viewer through an eyepiece optical unit so that the viewer can see the images. In such spectacles-type image display device, usually, the electronic images and the background images that transmit a spectacle lens are superimposed and displayed on an eyeball and, thereby a viewer can see the electronic images as translucent images, that is, see-through images in his/her field of view.
As a method of realizing see-through images, a device that uses a half mirror and lens or a concave mirror for an eyepiece optical unit and a device provided with a holographic optical element disposed on a spectacle lens (see, for example, JP2006209144 (A)) are known.
However, when electronic images are displayed in a see-through manner, if the environment is too bright, image light intensity of the background image will be too high and the light intensity of the electronic images will suffer from it, causing difficulty in viewing. So, if the luminance of the electronic images is increased in order to improve visibility, the luminance which is more than ten times the normal luminance is needed. Thus more power is consumed, which is economically inefficient. Therefore, in such environment, decrease in transmittance of the background images that are superimposed on the electronic images, in other words, decrease in the degree of see-through, is suggested to provide easy-to-see images.
For example, a spectacles-type image display device having an electrical switching means (see, for example, JP2830291 (B2)) that employs a liquid crystal shutter as a means for switching between see-through and non see-through or having a mechanical mechanism by which the background light is adjusted by mounting or dismounting a shielding member made of material which is a mixture of transparent material and colored material depending on the ambient brightness (see, for example, JP2001166703 (A)) is suggested.